The present invention relates to the beneficiation of coal by treating the coal with an aqueous ammonium salt solution and an organic solvent at a temperature from about 0.degree. C. to about 300.degree. C. for a time sufficient to remove minerals from the coal.
Many process have been developed to utilize coal directly in solid form by combustion, or by first converting it to useful liquids and gases. However, many of these processes suffer because of problems associated with the mineral matter contained in coal. For example, mineral matter in coal can cause recycle problems in liquefaction processes by causing the loss of hydrogen to form the undesirable side product H.sub.2 S. Mineral matter can also be a problem if the coal is burned directly because mineral matter contributes to slagging problems. Mineral matter in coal will also contribute to the release of sulfur dioxide as a pollutant. Consequently, it is highly desirable to remove as much of the mineral matter as possible before coal utilization.
Although various methods have been proposed for the removal of mineral matter from coal, there is still a need in the art for the development of more efficient and effective demineralization, or beneficiation, methods.
Much work has recently been done to develop efficient methods for acquiring useful liquids and gases from oil-shale as replacements for petroleum products. The inherent mineral matter in oil-shale represents significant debits in both handling and processing. Typically, the organic material which can be converted to useful fuels and chemical feedstock, preferably liquids, represents only about 10-25 wt.% of the oil-shale. Furthermore, during processing there is considerable increased abrasive wear on preparation and conversion plants owing to the nature of mineral matter contained in oil-shale. Also, relatively large reactor sizes are required to accommodate the higher material throughputs necessitated by the large quantities of mineral matter. Most important is the fact that large quantities of rock must be heated to relatively high temperatures in order to breakdown the kerogen of the oil-shale to liquid and gaseous products. This results in a loss in energy efficiency because much of this heat is not recoverable as useful work. Also, processes which include a catalyst are faced with a significant potential that the mineral matter inherently present in the oil-shale contributing to catalyst poisoning. Consequently, various processes have been developed over the years to beneficiate, or demineralize, oil-shale prior to further processing to concentrate the organic material and alleviate the above mentioned problems associated with oil-shale processing.
Although both physical and chemical methods have been developed for separating organic material oil-shale, thereby leaving substantially all of the mineral matter behind, chemical methods usually are superior over physical methods because of better separation obtained. One conventional type of chemical method for the separation of kerogen from oil-shale is acid dissolution of the inorganics. Such a technique typically involves the digestion of oil-shale with hydrochloric acid, hydrofluoric acid, or both. The hydrochloric acid dissolves the carbonate minerals and the hydrofluoric acid dissolves the silicate and clay minerals, thereby leaving a kerogen material containing about 5 to 10 wt% mineral matter. Although such schemes have met with varying degrees of success, they suffer the disadvantages of being corrossive and, in most cases, the inability to regenerate the acid reactants economically for recycle makes the process unattractive.
A variation in this acid technique is one disclosed by R. D. Thomas in Fuel, 48, 75-80 (1969) wherein all of the mineral carbonates are removed from Green River oil-shale by treatment with carbonic acid. Thomas discloses that after sodium hydroxide washing to remove fine clay-like particles, the carbonate-free material can be centrifuged in a saturated aqueous calcium chloride solution to yield a concentrate containing 85-95 wt% of the original kerogen with an ash content of less than 30 wt.%. Attempts to reproduce the physical separation results after demineralization with carbonic acid have been unsuccessful. See Vandergrift, G. F., Winans, R. E., and Horwitz, E. P., Fuel, 59, 634-6 (1980).
A variation of using acids to enrich the kerogen content of oil-shales which are rich in carbonates involves leaching by use of a sulfuric acid medium generated by the oxidizing capabilities of specific bacteria which convert pyrite in the oil-shale to sulfuric acid. The sulfuric acid produced decomposes the carbonate minerals in the oil-shale leaving a kerogen enriched material containing significant amounts of silica. See U.S. Pat. No. 4,043,884.
Other approaches to chemical beneficiation include extraction of low grade Western U.S. oil-shales with aliphatic amides to recover 69 wt.% to 83 wt% of the organics (U.S. Pat. No. 4,357,230), and treatment of various oil-shales with a 1:1 mixture of trichloro-isocyanuric acid and a chlorine donor such as calcium hypochlorite in the presence of a petroleum based naphtha solvent giving recoveries of organics up to about 98 wt% (U.K. Pat. No. 2,090,858).
Furthermore, disclosed in Intl. Patent Appl. Wo No. 82/03404 is a two-step treatment of oil-shale with 16 parts of fused alkali metal caustic at 350.degree. C. for 40 minutes followed by a sulfuric acid wash which led to recovery of 86 wt.% of the organics. This type of treatment suffers from the disadvantage that it may have a detrimental effect on the kerogen structure and reactivity.
Although various methods have been used, including physical grinding combined with chemical treatment, for beneficiating oil-shale, such prior art methods have not been able to effect a significant separation of mineral matter from the organic material under relatively mild conditions without the reagent being simultaneously and irreversibly destroyed. Consequently, there is still a need in the art for processes which are able to beneficiate oil-shale under relatively mild conditions and leave the benefication reagent in a form which is easily regenerable. Furthermore, none of the prior art methods suggest the use of ammonium salts for beneficiating oil-shales.